1. Field of the Invention
This invention relates to a process for preparing a dispersion liquid containing an organic, photoconductive azo pigment showing stable electrophotographic characteristics and a process for preparing in electrophotographic, photosensitive member.
2. Background of the Invention
Heretofore, electrophotographic, photosensitive members comprising an inorganic photoconductive substance, such as selenium, cadmium sulfide zinc oxide, etc. have been widely used.
On the other hand, electrophotographic, photosensitive members comprising an organic, photoconductive substance such as a photoconductive polymer represented, for example, by poly-N-vinylcarbazole, or a lower molecular weight, organic photoconductive substance represented, for example, by 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole, or combinations of these organic photoconductive substances with various dyes or pigments have been known.
Electrophotographic, photosensitive members comprising an organic, photoconductive substance have such advantages as a good film formability, a capability to form a film by coating, a high productivity, a low cost, etc. and further have such an advantage that the color sensitivity can be controlled as desired by selecting a photosensitizer such as a dye, a pigment, etc. to be used. Thus, they have been extensively investigated. Particularly owing to the recent development of a photosensitive member of the functionally separated type comprising a layer containing an organic photoconductive pigment as a charge-generation layer and a layer containing the aforementioned photoconductive polymer, low molecular weight organic photoconductive substance, etc. as a charge-transport layer, laminated to each other, remarkable improvements have been made to the sensitivity and the durability which have heretofore been regarded as the disadvantages of the conventional organic electrophotographic, photosensitive members, which improvements have promoted their practical applications.
Furthermore, various pigments applicable to the photosensitive member of functionally separated type, such as azo pigments, etc. have been also already found. It is also known that the sensitivity and spectroscopic characteristics of such an electrophotographic, photosensitive member depend upon the particle size and crystal form of the pigments as charge-generating substances.
According to a conventional process, a pigment prepared through a synthetic reaction is dispersed into solvent together with a binder by means of a Lall mill, sand mill or attriter over several hours to several ten hours to obtain a photoconductive composition (dispersion liquid). The process for directly dispersing the pigment into a solvent has a problem in obtaining a dispersion liquid containing uniform particles. This problem occurs owing to coarse particles being liable to precipitate in the dispersion liquid when wettability of the pigment is poor upon dispersing.
An electrophotographic, photosensitive member prepared from such an electroconductive composition containing coarse particles has not only reduced the number of carrier generations owing to a decrease in the trapping power, but also reduced carrier mobility owing to the increased void due to coarse particles, and furthermore has a deteriorated sensitivity such as reduced efficiency of carrier injections into the charge-transport layer owing to high roughness on the surface of the charge generation layer, etc.
The number of coarse particles can be indeed reduced by prolonging the dispersion treatment time of particles, but the already finely dispersed particles are excessively dispersed thereby and thus the coagulation state is liable to change and the particle side is also liable to change during or after the dispersing step, considerably deteriorating the stability of the dispersion.
In case of pigments whose crystal form transition proceeds in a dispersing solvent, the transition state of crystal form changes with a slight deviation in the dispersing conditions. Sometimes the stabilities of dispersion liquids with respect to time are diversified in not only sensitivity but also in the spectroscopic characteristics of the thus prepared electrophotographic, photosensitive members in each dispersion batch.